Driving MOSFETs in high-side applications often requires some means for level-shifting the signal and withstanding high voltage. A wide variety of isolated and non-isolated high side gate drive techniques, including direct, level-shifting, and bootstrap drivers, are described in Balogh, Design and Application Guide for High Speed MOSFET Gate Drive Circuits, Texas Instruments 2002.
A prior art bootstrap high-side drive circuit 11 is shown in FIG. 1 connected to drive the gate of high-side transistor 20 in power conversion circuitry 5. The circuit 10 includes high side driver 11 having VCC connection 12, ground connection 14, input connection 13, output 16, bias connection 15 and source connection 17. Ground-referenced control signals output by the controller 27 at control output terminal 28 are received on the input 13 of the driver 11 which is also referenced to ground 29. The driver output 16 provides a turn ON pulse to the gate of transistor 20 referenced to the source output 17 which is connected to the transistor source terminal 24. A bootstrap bias circuit, including bias capacitor 19 and diode 21, provides energy to power the driver output stage. The bias capacitor 19 is charged through diode 21 when the source terminal 24 experiences negative-going voltage transitions. A level shifter translates the ground (29) referenced input signal 28 to a relatively high voltage control signal for use by the output stage.
Typically the transformers, high voltage integrated circuits, opto-couplers or discrete components added to provide the necessary drive to turn the transistor on or off increase the cost and size of the drive circuitry. Many solutions result in lower switching performance to provide the requisite impedance or isolation between the drive circuit and the transistor.